Spontaneous panic spread through France in the summer of 1789, following the siege, on July 14, of the country's most hated prison. Two weeks after the storming of the Bastille, a first taste of the excitements that lay in store was granted to 19-year-old Lucie de la Tour du Pin. Out for a morning ride in rural Normandy, she came upon a bedraggled horseman who announced that Austrian troops were pillaging the next village. (Austrian invaders vied for the honours - as chief bogeymen - with British marines, a regiment of Swedes, a band of vengeful brigands in the pay of an enraged aristocracy, and the 30,000 Spaniards who were allegedly marching on Bordeaux.)

Du Pin, according to her own vivacious report, proved herself the heroine of the day. After preventing the village priest from tolling a summons to arms (Lucie wrestled his hands from the bell-rope), the young mother of two galloped off to investigate the - as it proved, unpillaged and equally petrified - neighbouring hamlet of Gaillefontaine. Having survived a particularly dodgy moment, when a villager fiercely identified her (on account of the equestrian's smart costume) as Queen Marie Antoinette, Du Pin took the good news back home.

For such a woman, saving two villages from pointless warfare was all part of a day's work. When she was 50, and in search of some escape from her grief - the guillotining of an adored father and the cruelly early deaths of four (out of five) beloved children - Du Pin started to write her memoirs. Published in 1907 (and never since out of print), they offer less of a self-portrait than an uncommon record of survival, by a resourceful woman who endured and outlived the most volatile period in France's history. Born into splendour in 1770, she spent her final 11 years sharing frugal lodgings at Pisa with her sole remaining child, who buried his mother there in 1853.

By her own account (Caroline Moorehead concurs) she suffered from a miserable upbringing. Her pretty, flirtatious mother died young; brought up by a tyrannical granny ("Her despotism ruled my entire life"), she retreated into bookish isolation. Later in life, Du Pin fiercely dismissed the suggestion that she had come to resemble her autocratic grandmother. "Despite whatever may have been said of me, I am not a domineering woman."

Not domineering? Moorhead's book underscores what leaps from every page of the memoirs; while her adored husband, Frédéric, was honest, reliable and brave, his wife could have made Ivan the Terrible quake in his boots. A tigress when it came to pursuing career appointments both for her husband and their eldest son, her steely courage served her best when she was fighting for the survival of her family. Who else would have had the nerve to demand favours from the sternly idealistic Jean-Lambert Tallien when he was presiding over the Bordeaux Terror in 1793?

Tallien rejected her appeal (while miming for her the act of decapitation). The intercession of his famously seductive wife, Thérésia, produced better results; passports were provided, enabling the Du Pins to set out for a new life in America.

It is possible to argue, as Simon Schama has done in Citizens, that Du Pin was never more at home than in America, and that - had she not been a loyal wife, devoted to furthering her husband's career - she would have remained there. Talleyrand, encountering his fellow-exile as she chopped mutton for the spit, was impressed by her grace and fortitude. She seems to have been content. She milked cows, sold her hand-churned butter, made friends with the local Iroquois and took care to set free, publicly, the four slaves who worked at her home.

Schama thinks that Lucie relished rural solitude; Moorhead ("Slowly, very slowly, the days passed") reasons that such a vibrant woman must have ached for the greater sophistication of Europe. In 1796, whether in joy or resignation, she returned to France, to begin the task of reassembling the looted contents of her husband's abandoned château in Bordeaux. Following the defeated monarchist coup of 1797, she once again entered exile: in England.

Many journeys would lie ahead in her life, but it was at this point - as Moorhead describes everything from the Hanoverian fashion for wearing false buttocks at court to the regularity of the mail service to Richmond - that this reader began to flag.

There's no doubt that Du Pin's story merited recovery and examination. The book's problem arises from the decision to place this story within a larger context. Moorhead is not primarily a historian and the background she provides, while outstanding for the wealth of its detail, floats free of any unifying thesis. The narrative, whenever it returns to Du Pin herself, becomes vivid and absorbing; too often, however, the central figure disappears from view behind a mighty canvas of events, incidents and descriptions. The reader is left floundering somewhere between astonishment (how did this diligent author unearth so many facts?) and, sadly, fatigue.